[simple] introduce page_with_header.html template

Previously the preferences & stats templates contained the markup:

<a href="{{ url_for('index') }}"><h1><span>SearXNG</span></h1></a>

There are many things wrong with this:

1. the markup was duplicated

2. the CSS needed to be changed whenever a new page wanted to use this
   header (since the CSS used page-specific selectors)

3. h1 should be reserved for the actual page title
   (e.g. Preferences or Engine stats)

4. the image was set via CSS which also set:

       span { visibility: hidden; }

   which however removes the alternative text from the accessibility
   tree (meaning screen readers will ignore it).

This commit fixes all these problems.

searx/templates/simple/preferences.html

@@ -1,7 +1,7 @@
 {% from 'simple/macros.html' import tabs_open, tabs_close, tab_header, tab_footer, checkbox_onoff, checkbox %}
 {% from 'simple/icons.html' import icon_big %}
 
-{% extends "simple/base.html" %}
+{% extends "simple/page_with_header.html" %}
 
 {%- macro plugin_preferences(section) -%}
 {%- for plugin in plugins -%}
@@ -95,10 +95,7 @@
 
 {% block head %} {% endblock %}
 {% block content %}
-
-<a href="{{ url_for('index') }}"><h1><span>SearXNG</span></h1></a>
-
-<h2>{{ _('Preferences') }}</h2>
+<h1>{{ _('Preferences') }}</h1>
 
 <form id="search_form" method="post" action="{{ url_for('preferences') }}">